To set the issue's assignee from e-mail, set the ccassignee name parameter in the comment handler for the POP/IMAP service used to create issues. Refer to Creating Issues and Comments from Email for more information.
The JIRA Advanced Mail Handler allows users to define fields for the issues created email such as reporter, issue type, priority, summary, description and more.
The priority can be set on a scale 1-5 by setting the X-Priority on the email itself. See your mail server's documentation for tips on how to set the X-Priority. A related discussion appears in JRA-7316.